O. F. Schirmer

O- trapped holes in acceptor doped KNbO3

Abstract Under illumination with visible and near UV light, Ti doped KNbO 3 shows ESR of O- trapped holes as well as a corresponding optical absorption band with maximum at ∼ 2 eV. In the most likely model the hole wavefunction extends over two neighbouring oxygen ions next to Ti 4+ substituting for Nb 5+ .

The paramagnetic properties of CO2+ in LiNbO3 and LiTaO3

Abstract ESR of Co2+ in LiNbO3 is observed at g∥=2.59 and g⊥=5.04 with A∥≈0, |A⊥|=154⋅10-4 cm-1. Similar values apply to LiTaO3:Co2+. The ESR lines broaden exponentially at T > 13 K indicating an Orbach relaxation via a state ∼100 cm-1 above the groundstate, lower than predicted by reasonable spin-orbit and trigonal field parameters. This level depression is attributed to a dynamic Jahn-Teller effect. The axial Co2+ spectra are accompanied by lower intensity lines having nonaxial symmetry.

An off-center ion near a Ba site in BaTiO3 as studied by EPR under uniaxial stress

Abstract In BaTiO3 a paramagnetic defect is identified by EPR which is attributed to a paramagnetic S=1/2 ion off-center near a Ba site. Twelve magnetically distinguishable sites contribute to the angular dependence pattern of the spectra, each of the sites being characterized by an orthorhombic g-tensor. The relative occupations of the sites change under uniaxial stress in a manner consistent with the model. The features of the center are highly characteristic for the indicated off-center position, as revealed by comparison with the previously identified Ti3+ near a Sr2+ site in SrTiO3. On the basis of the principal values of the g-tensor the paramagnetic ion can be identified with Ni. It is discussed that Ni1+ is the most likely charge state.

Jahn–Teller and off-center defects in BaTiO3: Ni+, Rh2+, Pt3+ and Fe5+ as studied by EPR under uniaxial stress

Application of uniaxial stress reveals the Jahn–Teller (JT) properties of the defect ions Ni+ (3d9), Rh2+ (4d7) and Pt3+ (5d7), all incorporated on Ti4+ sites in BaTiO3. In all cases the vibronic ground states are stabilized by an E e Jahn–Teller effect, here leading to tetragonally elongated defect–O6 octahedra. Orbitals of (x2−y2) type are thus lowest for the hole-like Ni+, and (3z2−r2) for the electron-like strong-field cases Rh2+ and Pt3+. The three systems are characterized by motionally averaged isotropic spectra at elevated temperatures. At low temperatures strong dependences on the distribution of internal strains are found, leading to a coexistence of isotropic averaged and quasi-static tetragonal situations for Ni+, becoming more static under uniaxial external stress. The latter behavior is also identified for Rh2+. For Rh2+ and Pt3+ the coupling of the JT systems to the external stress could be determined quantitatively, predicting rather low JT energies by a simple model calculation. The observation of tetragonal distortions of these defects thus points to the presence of regions with strong internal strains. The impurity ion Fe5+ (3d3), also replacing Ti4+, is found to go spontaneously off-center along [111] type directions. The defect axis orients parallel to the stress direction, indicating an axially compressed Fe5+–O6 oxygen arrangement in the off-center situation.

Oxygen vacancy related defects in batio3

Abstract Depending on the acceptor content of BaTiO3 crystals, two types of Ti3+ ESR signals are observed. Acceptor-poor samples, which thus also have low concentrations of compensating oxygen vacancies, Vo, contain isolated Ti3+ free small polarons. Acceptor-rich crystals show signals to be attributed to Ti3+ next to Vo—alkali-acceptor complexes. This model is supported by a comparison of the optical absorptions and light induced charge transfer transitions of both types of specimens.

Phase separation in La2CuO4+δ as probed by impedance spectroscopy

Abstract The dc-conductivity and the high-frequency capacity of a La2CuO4+δ single crystal are determined by impedance spectroscopy in the frequency range 20Hz – 1MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantities is studied for E parallel and perpendicular to the CuO2-planes as well as for different oxygen doping levels, δ, of the sample. The results are consistent with the model of a diffusion controlled formation of a metallic- and below TC superconducting - percolation network in an insulating background.

Phase Separation in La2CuO4+δ by electrical transport measurements

The dc conductivity and the high-frequency capacitance of a La2CuO4+δ single crystal are determined by impedance spectroscopy in the frequency range 20 Hz to 1 MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantitites is studied for E parallel and perpendicular to the CuO2 planes as well as for different oxygen doping levels,δ, of the sample. The results are consistent with the model of a diffusion-controlled formation of a metallic—and belowTc superconducting—percolation network in an insulating background.